1. Field of the Invention
The present invention relates to an ophthalmologic apparatus for acquiring a tomographic image of an eye to be inspected, and an ophthalmologic observation method of observing the eye to be inspected.
2. Description of the Related Art
At present, various different types of optical equipment are used as ophthalmic equipment. Of those, an eye can be observed with various types of equipment including an anterior ocular segment photographing device, a fundus camera, a confocal scanning laser ophthalmoscope (SLO), and an optical coherence tomography (OCT) apparatus that is an optical coherence tomographic imaging apparatus using optical interference of a low coherence light beam. In particular, optical tomographic imaging apparatuses are designed to obtain a high resolution tomographic image of a fundus, and are currently on the way to becoming an indispensable apparatus in outpatient clinics specialized in diagnosis of the retina. Hereinafter, optical tomographic imaging apparatuses including such OCT systems are referred as an OCT apparatuses.
In operation an OCT apparatus applies a low coherence light beam to a retina and measures reflected light from the sample (retina) with a high degree of sensitivity, by using an interferometer. Further, the OCT apparatus can obtain a tomographic image by scanning the low coherence light beam over the sample. In particular, the tomographic image of a retina is widely used in ophthalmic diagnosis.
In recent years, OCT apparatuses conventionally operated according to a, so-called, time domain method, in which a tomographic image is acquired by performing scanning along an optical path length of a reference beam in each tomographic acquisition position. However, OCT apparatuses have made a shift to a Fourier domain method of operation that eliminates the need for scanning along the optical path length of the reference beam in each tomographic acquisition position. Therefore, it is now possible, using the Fourier domain method, to acquire the tomographic image at a faster speed than before. The increase in the speed allows three-dimensional image taking of the fundus to be carried out, which has been difficult with the time domain method.
However, in three-dimensional imaging, two-dimensional scanning of the fundus is necessary even with the Fourier domain method, and hence the influence of an involuntary movement of an eye being scanned cannot be ignored. The influence of eye movements can be reduced by detecting the eye movements at the time of scanning and correcting a low coherence light beam scanned over the retina in synchronization with the detected eye movement. The eye movement can be detected by taking a fundus image of the eye in parallel with the image taking performed by the OCT apparatus and detecting displacements of characteristic points within the fundus image.
A system for acquiring the fundus image is divided into a one-time imaging system using the fundus camera and a scanning imaging system using the SLO. In general, when the scanning imaging system is used rather than the one-time imaging system, high contrast and high resolution may be realized. Therefore, in order to perform high-precision eye movement detection, the scanning imaging system is used. However, in the case of the scanning imaging system, a time lag occurs in a single acquired image. In other words, there is a time difference between the start and end of imaging. Thus, when the scanning imaging system is used to acquire an image of a fundus which is continuously moving, the acquired image distorts. In order to accurately detect the movement of the fundus, it is necessary to cancel the distortion in the single image.
Japanese Patent Application Laid-Open No. 2006-235786 discloses an invention for calculating the shift amounts of a plurality of images acquired by a scanning imaging system to perform position correction. In order to calculate the shift amounts, five parameters, that is, translation factors (Δx,Δy), a rotation factor (θ), and magnification factors (βx,βy), are used.
However, the object of the invention disclosed in Japanese Patent Application Laid-Open No. 2006-235786 is to align images, each having a predetermined shift direction, into a single image, but does not describe any method for accurately detecting a movement of an object, for example, a complicated movement of an eye to be inspected.